The invention is about security documents and articles, i.e. bank notes, value papers, identity documents, cards, tickets, labels, security foils, security threads and the like, which can be easily authenticated by stationary or portable electronic equipment, such as currency acceptors, processing machines, ticket validators, hand-held authentication devices, etc., and of methods to produce and authenticate such security documents or articles through the application of dyes or pigments exhibiting a specific spectral absorption.
The increasing availability to the general public of desk-top color printing and reproduction equipment, such as color computer printers, color scanners and color copiers, has raised an additional threat to document security. In particular, the currently used security features do not provide sufficient protection against counterfeits at automatic banknote acceptors, where exclusively electronic means are used for the authentication.
Known security features suitable for authentication with electronic means are described for example, in EP-B-0 05 and EP-B-0 053 183, EP-B-0 05, EP-B-0 024 053, EP-B-0 34, or EP-B-0 537 431. These features are based on luminescence, magnetic properties, or on absorption in the non-visible range of the electromagnetic spectrum. However, the exploitation of several of said physical effects is accompanied with drawbacks. For example, in the case of luminescence, the to-be-measured light intensities are generally low, requiring sophisticated technology, including the shielding of ambient light and of other potentially perturbing effects. Magnetic properties are less suited as a security feature, because they require in general a mechanical contact between the bank note and the magnetic sensor device (reading head). This constitutes a major source of errors, caused by jamming of “in-bad-shape” bank notes in the reader device.
A shortcoming of the “classic” UV or IR-absorber based banknote security features, such as disclosed in EP-A-0 024 053, is noteworthy that they rely on the use of a single absorber compound as a covert security element outside the visible range of the spectrum. The presence of the covert security element can be revealed with the help of commonly accessible, cheap CCD cameras (e.g. door-watcher cameras, having a sensitivity range of 300 to 1,100 nm wavelength) and commonly accessible generic UV and IR filters, available in photo-shops. Generic UV- and IR-absorbing compounds are furthermore used in many areas of current technology, noteworthy in certain electrophotography toners, and are thus commercially available as well.